Object.getOwnPropertyDescriptors(O) ? // plural

[C. Scott Ananian]

I was also a bit puzzled by the iterator return value.  I expected
that it would handle mutation (adding new properties) during traveral
the way that the `Map` and `Set` iterators do.  I was surprised to see
that all the users turned the iterator into an array; none seemed to
handle concurrent mutation.  So why not return an `Array`?
  --scott